This invention relates to optical waveguide propagation structures and in particular to such structures incorporating compensation for effects of temperature on operation.
1. Related Applications
The subject matter of the invention disclosed and claimed in this application is particularly suited to employment in conjunction with the optical waveguide transmission device disclosed in copending application Ser. No. 09/616,487 of Henryk Temkin and Rudolf F. Kazarinov, filed Jan. 14, 2000 (U.S. Pat. No. 6,493,487, issued Dec. 10, 2002) and assigned to the assignees of the present application. The disclosure of application Ser. No. 09/616,487 is hereby incorporated by reference into this application as if it had been fully set forth herein.
2. Background of the Invention
It is known that changes in ambient temperature can adversely affect operation of optical waveguide structures, for example wave division multiplexers/demultiplexers. In a particular example, silica based optical waveguide structures employ a doped silicon dioxide core having a higher index of refraction than surrounding silicon dioxide cladding. The effective index of refraction of waveguides changes with ambient temperature, resulting in a spectral shift of the channel wavelengths by about 0.0012 nm per degree C. while the required precision of the channel wavelength is about the same. Thus, it is desirable that the temperature variation during operation of such a waveguide structure should not exceed one degree C.
It has been proposed to use thermoelectric coolers or heaters to reduce ambient temperature variations during operation of such optical waveguide propagation structures. However, such measures have added significantly to overall cost and structural complexity.
Accordingly, there is a recognized need for a simpler and more elegant solution to the problem of reducing adverse effects of temperature on optical waveguide operation, in reducing wavelength changes with temperature of optical signals propagating along optical waveguides, and in particular in optical waveguide grating structures employing a multiplicity of optical waveguides.
According to the present invention, a folded optical waveguide structure comprises a substrate supporting a waveguide slab and an array of laterally spaced grating waveguides extending from the slab along the substrate to propagate optical signals to and from a reflective surface of a mirror member disposed at an end face of the substrate. A thermally conductive body is interposed between the mirror member and the substrate such that dimensional changes of the body resulting from changes in ambient temperature, tilt the mirror member with respect to the grating waveguides at said end of the substrate. The thermally conductive body is so dimensioned and has a thermal coefficient of expansion such that temperature induced changes in wavelengths of optical signals propagated along the grating array waveguides and reflected from the reflective surface of the mirror member are substantially compensated by the tilting of the mirror member with respect to the grating waveguides at said end of the substrate.
A layer of thermal matching material may be located between the end of the grating waveguide array and the reflective surface, and the tilting of the mirror member changes the optical path lengths between the grating waveguides and the reflective surface of the mirror member to compensate for temperature dependent changes in optical path lengths along the grating waveguides.
Conveniently, the thermally conductive body may be a metal body, for example copper or aluminum, and in one embodiment is supported by the mirror member, for example, mounted in a recess in the mirror member. Alternatively, the thermally conductive body could be mounted by the substrate or partly by both the mirror member and partly by the substrate to permit the required amount of tilt of the mirror member relative to the substrate.
Advantageously, thermally conductive body is laterally offset from one side of the grating array and the mirror member tilts about an axis offset from the opposite side of the waveguide grating array.